1. Field of the Invention
This invention relates generally to pipe tongs used for gripping and threadably joining lengths of pipe and more particularly to dies used therein for gripping chromium pipe without significant marring or otherwise causing surface fracturing of the pipe.
2. General Background
In the oil gas industry pipe tongs are used primarily for threadably engaging and disengaging tubular goods, such as drill pipe or production tubing and the like. Such tongs generally comprise a set of circumferentially spaced bodies called dies, which are rigidly held in a rotatable body which surrounds the locus of the drill pipe body. By means well known within the art, the device can be manipulated into position about the circumference of a length of pipe in a manner whereby the inner sides of the dies, having hardened metal gripping teeth, bite into and frictionally engage a portion of the pipe to be threadably engaged with or disengaged from a second length of pipe. While one of the pipe lengths is retained, the dies within the tong conform with the unrestrained pipe and are camed into locking engagement with the pipe body. The dies and their retaining bodies are then power driven to either engage or disengage the threaded pipe bodies. Such tong dies are available with various tooth configurations which help grip the pipe. Such configurations include transverse mud grooves, which allow the pipe dies to maintain a grip even in contaminated conditions, such as when the pipe is coated with mud and oil. However, it is well known in the art that damage, to the pipe occurs when the dies wear unevenly or when the die teeth become damaged producing jagged edges, in which case stress risers may be set up in the surface of pipe which may result in premature pipe failure. The accepted method of gripping pipe in this manner depends on the ability of the die teeth to penetrate the surface of the pipe to some degree rather than applying excessive force, which may crush or misshape the pipe. This problem is compounded when such dies are used on high chromium pipe. Chromium or other nickel alloy pipe is often used in highly corrosive wells, such as Hydrogen Sulfide (H.sub.2 s) gas wells. Such pipe is expensive and must be handled carefully to avoid damage to the chromium surfaces which attracts corrosion, thereby leading to early pipe failure. Therefore, a new and better means of gripping such chromium and nickel alloy pipe during the connection make-up or break-out procedure is required in order to prevent damaging the chromium pipe surfaces. A problem also exists when the hardened, high carbon, steel teeth on the dies make contact with the chromium or nickel alloyed pipe, thereby exerting high contact pressure. It has been found that such high carbon steel dies tend to transfer small amounts of carbon to the pipe at each penetration point. Such carbon transfer spots have been found to set up sites for corrosion which lead to stress cracks in the pipe. It has been found that carbon creates galvanic action, thereby hardening pipe in the same manner as hydrogen sulfide, causing brittleness of the metal.
Tests on chrome pipe with salt spray have shown that any discontinuity in the surface of the pipe causes a deterioration of between 0.011-0.015 loss in pipe wall thickness per year. For example, a number 13 chrome pipe having 0.217 wall thickness with a 0.028 penetration coupled with 0.015 corrosion factor per year accelerates corrosion deterioration exponentially.
Others in the art have attempted to address the problem of handling chromium pipe in a manner designed to reduce penetration, such as that disclosed by U.S. Pat. No. 5,451,084 wherein strips having hard teeth which get progressively softer along its length are held in a resilient base to allow flexibility. However, such structures fail to address the problem of sharp tooth edges resulting from mud grooves cut vertically through the tooth configuration and the problem of carbon transfer to the pipe body.
Slip, elevator and tong dies all rely on the biting action of the die's teeth into the pipe body for griping the pipe. However, recently the industry has begun addressing these problems by attempting to reduce stress induced into the surface of the pipe through better fits, flexible die seats, etc. However, to date, slip dies still generally produce penetrations of between 0.017-0.028 of an inch with pipe loads of 14000 ft. with up to 100% carbon transfer. Tests show that pipe marred by pipe dies have penetration depths of up to 0.075 of an inch and consistently result in high carbon deposits in the penetrations of the pipe. Therefore, when such pipe is used in high corrosive wells, they last only a few weeks. However, the industry still considers die penetration of the surface of the pipe to be a necessary evil. However, it is becoming essential that such penetration by the die teeth into the pipe body must be kept to a minimum, generally in the order of less than 0.002/1000 of an inch. Many of the prior art dies have been found to have numerous chips and cracks on the teeth prior to use as a result of the heat treating and handling process. Such pre-use chipping, as well as the normal use chipping of the teeth, results in sharp edges which cut and mar the pipe surface even further and increase the amount of carbon deposited in the pipe penetrations.